Okay, I know this is remedial, but please bear with me, and maybe it will be useful to a few others (okay, probably just me) to know the answers:

1) is it better to do a compression test with cold or warm engine; I've read advocates for either side.
2) in doing the compression test, do you kick a set number of times (e.g., 5), or do you keep kicking until the compression reading no longer increases? Again, I've read differing opinions elsewhere on this.

Now the reason behind the questions: My bike has slowly but surely over the last 3 months become harder to start from cold - not awful, but 4-5 kicks have become more like 12-15. I was thinking colder weather, bike sits until weekend, not a major concern. Then for the last month or so, it sometimes doesn't want to start after I stop for gas. Feels like very little compression. But I can bump start it no problem and off I go, running fine. So I thought, check compression. I did this with the engine warm. At 5 kicks, both cylinders read 110 or so. Keep kicking another 5-7 times, and it builds to 117/120, which seems okay. So does this suggest rings are okay? I have 12,000 km on these rings since a complete new top end. What is the approx. service life on rings for basic street riding? (I could swear I remember Andy posting something on this, but I can't for the life of me find it doing multiple searches!)

If not the rings, any other thoughts on possible causes of the hard re-starting? Answers to any or all these questions would be greatly appreciated.
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-Jim
'93 MC21

I have always done my compression tests by continuing to crank till the needle stops. Throttle bodies should be full open. But I really don't have that much experiance with 2-stokes. You should also consider a leak-down test. I don't take any one test or reading as gosphel. I look at all the evidence and base my descision on all the facts as I understand them.
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james
Its not how fast you go...Its how you go fast.
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compression will read slightly higher when the engine is warm.
also it is more accurate if you take the other cylinders plug out.

I would consider 12,000km on rings quite a bit and personaly would be replacing them (though it does seem to have good compression).

Andy's bike had almost identical problems untill we got inside it and found dead seals etc.

However Andys bike has never really been a good kick starter but bumps with ease. Mine on the other hand pretty much always goes second kick unless I dont turn the fuel on
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If I have to take the carbs off once more...

okay, thanks for responses. I did do the compression test with engine warm, throttle wide open, both plugs out, and kicking until the needle no longer moved, so I guess the 117/120 reading I got should be reasonably accurate. And yes, I was wondering if, given the okay compression, this might be a sign of tired or worn out crank seals. I know there are 22,000km on the crank seals, as they were replaced when I got this motor secondhand, but I don't know how many miles were on the crank as a whole (bearings etc.) before I got the motor from Tyga (though they rated it an "A" engine).

So given this history, it seems I should replace the rings at a minimum. The question I have is whether I should just bite the financial bullet and replace the crank now too, or just try replacing the rings and see if that alone solves the increasingly hard starting problem. (Bear in mind that I don't like spending money before it's time, but I also don't want to save money now if it involves a significant risk of a crank failure that will take out the top end too, if you see what I mean.) Opinions on rings only versus total rebuild? Thanks.
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-Jim
'93 MC21

To be honest I dont think just replacing your rings will solve the trouble, as said before your compression seems quite good anyway. However I would be fitting rings anyway
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If I have to take the carbs off once more...

have you tried changing plugs? don't jump the gun, work on the crank isn't cheap.

Yes, the plugs are another angle on this. I changed the plugs after the first time it wouldn't restart after stopping for gas. The plugs looked pretty carboned up, not wet fouled but "crusty" carbon. I have been purposely running the main jetting a few sizes richer than a 2-stroke tuner had recommended (128 instead of his recommended 122) because I was paranoid about being too lean. The ends of the stingers also suggest I'm perhaps too rich. So one theory was that the plugs slowly fouled. But putting new plugs in didn't noticeably improve the hard starting. I'm thinking because it is waaay cheaper than getting into the crank (and something I can actually do myself), I might still first try going down one size on the mains.

Assuming that doesn't do anything, and I replace the rings simply because of the miles on them, is there anything else (cheaper) than the crank that anyone can think of? Assuming not, and assuming it is therefore dodgy crank seals, any sense of what the risks are of trying to squeeze some more street running time out of the crank, i.e., is it likely to go suddenly and take the top end with it, or is it more of a slow death, where it just keeps getting harder and harder to start to the point that I can't stand it any longer? Lots of questions and assumptions, sorry, but I'm just trying to find my way with 2-strokes. Thanks.
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-Jim
'93 MC21

Hi Jim
Have you tested your battery with a load tester or tried another just to see if this improves the starting ? I keep mine on an optimate, seems to make them last for ages.

Thanks, yes, in fact in addition to putting in new plugs, I replaced the battery with a new one when the hard-starting, er, started (is that an oxymoron?!). I too keep it on a battery tender all the time to good effect. I also have a neat little continuous digital display of the voltage that has shown normal values and suggests the voltage regulator is doing its job. Thanks.
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-Jim
'93 MC21

well if you eliminated electrical, ( plugs and battery) I'd have a look at the carbs. the float level could be off a bit or even stuck.

Keep these ideas that don't involve having to replace the crank coming. I'll check into that, thanks!
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-Jim
'93 MC21

nsr 250 needs the air fuel mixture pretty near spot on if the carbs run rich and it is starting to go low on compresion it will be hard to start my mc 21 se is sometimes hard to kick when cold and has only 4500km from new on the clock and compresion is 120 even adirty air filter will cause bad starting

hmmm, it has been a good while since I peeked at the air filter. Definitely worth a try. It would sure be nice if it were something so simple. Thanks very much!
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-Jim
'93 MC21

I kept on telling myself that too Jim!

Whip the flywheel cover off and give the flywheel a good man-handle! Sit on the bike and reach down, and rock the flywheel up and down. Turn the flywheel 30° and repeat until it's gone through 360°. It's not the definitive test, but you can feel even very tiny play. Any play at all would suggest the crank is on the way out, and if the crank is on the way, then the centre seal is almost certainly shagged!

Sorry, but it how it is! Mine went on for ages, and right up until just a couple of months before we pulled it apart it would scrape 60hp, but it was rough as hell!

Goes like a rocket now it's rebuilt mind you!!
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Andy.
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